A. Field
This invention relates to hermitic seal arrangements between residual limbs and vacuum retained prosthetic sockets.
B. Related Art
Residual limbs of amputees are inserted like a piston in a cup-shaped prosthetic sockets. Thus, it is known to utilize atmospheric air pressure (or partial vacuum) to retain such prosthetic sockets on residual limbs. A residual limb for this purpose must be kept hermetically sealed against the inner prosthetic socket wall or against a liner pulled over the residual limb.
With proper sealing, a force tending to remove the prosthetic socket from the residual limb will generate a partial vacuum that reacts against the force. On the other hand the retention force collapses immediately when air enters a gap between the residual limb and the prosthetic socket.
The European patent application EP 0 632 765 discloses a prosthetic socket fitted with a seal which maintains sealing between the residual limb and the prosthetic socket.
For that purpose the prosthetic socket is fitted with a peripheral groove located a distance from its distal end and which receives a sealing ring. The sealing ring consists of an annular core covered with a layer of elastomer. A sealing lip is formed on said ring and extends toward the inside area of the prosthetic socket.
The purpose of the sealing lip—which hugs the skin of the residual limb is to compensate for the diameter fluctuations of the residual limb. Because the prosthetic socket is made of a rigid material, its inside diameter is constant. On the other hand, the residual limb volume changes over the long-term and the short-term. The long-term fluctuations are caused by changes in tissue volume, whereas the short-term fluctuations depend on the residual limb's blood pressure and circulation within the residual limb. It is known that at higher temperatures the extremities tend to swell while at low temperatures their diameters shrink. This phenomenon may be observed for instance even in a healthy individual at his watch band.
The lip seal is folded toward the closed prosthetic socket end when the patient inserts a residual limb into the prosthetic socket. The lip seal will then rest against the residual limb solely on account of being prestressed.
The moment a force is applied to remove the prosthetic socket from the residual limb, a slight partial vacuum is set up between the residual limb and the prosthetic socket space between the distal end and the lip seal. The magnitude of the said partial pressure is approximately proportional to the extent of the annular gap between the residual limb and the prosthetic socket.
With respect to the prior art, the pressure difference at the lip seal is in a direction lifting said lip seal from the residual limb, thereby allowing air to enter the annular gap and, on account of the ensuing loss of partial vacuum, retention is lost.
The magnitude of the admissible partial vacuum at which the sealing lip would detach depends on the radial prestressing force by which the sealing lip rests against the residual limb. It is observed that a comparatively large prestressing force is required, which in turn may entail interference in the blood circulation in the region between the sealing lip and the distal end of the residual limb.
Accordingly, the objective of the present invention is to create a seal the sealing effect of which is substantially independent of a prestressing force magnitude.